1. Field of the Invention
The present invention relates generally to thermal sensors and more particularly to thermal sensors for thermography.
2. Description of the Related Art
Conventional breast cancer screening techniques are based on detection and evaluation of tissue mass structure. X-Ray mammography is commonly uses for breast cancer screening. Several drawbacks of X-Ray mammography include: (a) exposure of a population to systematic doses of radiation; (b) reduced effectiveness of X-Ray mammography in the case of dense breasts (a relatively common condition among young women); (c) reluctance of some women to undergo the uncomfortable procedure; and (d) inability of X-Ray mammography to detect early tumors of small size. For detection, the mass must be of a minimum size.
It has been shown that angiogenesis (the recruitment of new blood vessels from existing vessels) is a necessary condition for all solid tumors to grow beyond a diameter of several millimeters (J. Folkman, New England J. of Medicine, 285:1182-1186, 1971; N. Weidner et. al. , New England J of Medicine, 324:1-8,1991). The increased blood vasculature and blood perfusion associated with early tumor growth together with the increased metabolic output of tumors that are growing are hypothesized to lead to higher tumor temperatures than found for surrounding tissues. This hypothesis has been verified in a qualitative manner for tumors that are near the surface of skin in the case of breast cancer. Thus, thermographic sensing with an appropriate technique that is sensitive enough and is free of errors caused by other environmental factors would be useful as a screening method for cancer, particularly for breast cancer.
In applications such as aforementioned U.S. Pat. No. 5,909,004, thermographic sensors are used to measure thermal temperature gradients. To minimize the perturbation of the thermal temperatures being measured, the sensor must have minimal lateral thermal conductivity. In addition, the thermal time constant and mass must be minimized so that rapid reading of the temperatures can be accomplished. In the field of thermography, the surface temperature of the human body is mapped to provide information that is indicative of early tumor growth.
Infrared (IR) cameras or imagers have been used to attempt to measure small temperature gradients. An example of a thermoelectric infrared detector array can be found in Sclar, U.S. Pat. No. 4,558,342. Measurements of temperature distributions in human tissue to detect tumors must map the surface temperature accurately so that any contribution from internal tumors can be properly detected. Infrared sensors (which cannot be in direct contact with skin) can yield unreliable temperature data due in part to effects of ambient air flow on the skin surface and due to variations in skin emissivity and orientation relative to the infrared sensor. Skin emissivity can be a particularly insidious problem in practice and varies due to the presence of a variety of substances on the skin such as oil, water or particulate debris. Variations in emissivity cause apparent changes in temperature as sensed by the infrared camera that mask the underlying true temperature gradients on the skin.